1. Field of the Invention
The present invention relates to a video signal processing unit used for a monitor, and more particularly, to a circuit and method for adjusting the width of a fly-back pulse, the apparatus and method being applied to a video signal processing unit realized in one chip.
2. Description of the Related Art
In general, a horizontal blank signal used for the purpose of blocking an electron beam during a horizontal retrace period of a monitor system is generated by receiving a fly-back pulse from an external horizontal deflection transformer. Conventionally, a video amplifier, an on-screen display (OSD) using a horizontal blank signal, and a horizontal/vertical synchronous signal processing unit using a fly-back pulse have been manufactured as separate chips.
FIG. 1 is a schematic block diagram illustrating a video signal processing unit where a conventional circuit for adjusting the width of a fly-back pulse is applied. The video signal processing unit includes a horizontal deflection transformer 10, a fly-back pulse width adjustment circuit 12, a video amplifier 14, an OSD unit 16, and a horizontal/vertical synchronous signal processing unit 18.
The fly-back pulse width adjustment circuit 12 illustrated in FIG. 1 receives a fly-back pulse (AFC) from the horizontal deflection transformer 10, adjusts the width of the fly-back pulse, and outputs the adjusted pulse as a horizontal blank signal (H_BLK). The pulse width of the horizontal blank signal must be adjusted to agree with a retrace period of a horizontal deflection yoke. In most cases, the fly-back pulse is applied after a predetermined time delay, and consequently, the fly-back pulse has a wider width as compared with a real horizontal retrace period. Therefore, a conventional video signal processing unit has used a fly-back pulse which is received from the fly-back pulse width adjustment circuit 12, the width of which is adjusted to properties of a monitor, as a horizontal blank signal. If the width of the horizontal blank signal is greater than the horizontal retrace period, the horizontal blank signal can invade a video signal region. On the contrary if the width is less than the horizontal retrace period, there may be interference in the video signal region caused by electron emission from an electron gun of a cathode ray tube (CRT). Therefore, the width of the blank signal must be adjusted appropriately.
FIG. 2 is a detail circuit diagram illustrating the fly-back pulse width adjustment circuit 12 illustrated in FIG. 1. In this figure, the fly-back pulse width adjustment circuit includes capacitors C21, C22, and C23, resistors R21, R22, and R23, a diode D21, and a transistor Q21.
With reference to FIG. 2, each of the capacitors C21, C22, and C23 has a very large capacity, because the fly-back pulse (AFC) is applied with a high voltage of several kilovolts. Accordingly the capacitors C21, C22, and C23 cannot be built as one chip and must be provided as external components. Moreover, in the case of the fly-back pulse width adjustment circuit 12 of FIG. 2, the capacitors C21, C22, and C23 and the resistors R21, R22, and R23 have been set to have fixed values, so that there is a disadvantage that the width of a pulse is fixed. Consequently, it is difficult to manufacture a printed circuit board (PCB) with use of the fly-back pulse width adjustment circuit 12 which is provided as external components, thereby causing an additional increase of the manufacturing cost. In addition, there is another disadvantage that the width of a fly-back pulse must be adjusted to accommodate properties of newly introduced monitors.
To solve the above problems, it is an object of the present invention to provide a circuit for adjusting the width of a fly-back pulse which is built into a video signal processing unit realized as one chip and which can adjust the width of a fly-back pulse easily.
It is another object of the present invention to provide a method for adjusting the width of a fly-back pulse which is performed in the fly-back pulse width adjustment circuit.
Accordingly, to achieve the first object of the invention, there is provided a circuit for adjusting the width of a fly-back pulse which is applied in a video signal processing unit realized in one chip according to the present invention. The fly-back pulse width adjustment circuit is built into a video signal processing unit including a video amplifier, and on-screen display, and a horizontal/vertical synchronous signal processing unit within the video signal processing unit realized as one chip. The fly-back pulse width adjustment circuit generates a horizontal blank signal by adjusting the width of an external fly-back pulse, and comprises a pulse standardization unit which shapes a fly-back pulse input from the outside via an input terminal and applies the shaped fly-back pulse to the horizontal/vertical synchronous signal processing unit. A pulse width adjustment unit adjusts the width of the shaped fly-back pulse in response to a predetermined control signal, generates a horizontal blank signal whose occurrence time can be varied from the result of the adjustment in response to a selection signal into a horizontal blank signal occurring at a different time, and then applies the horizontal blank signal to the video amplifier and the on-screen display unit.
To achieve the second object of the invention, there is provided a method of adjusting the width of a pulse which is applied to the video signal processing unit realized as one chip according to the present invention. Preferably, the method of adjusting the width of a pulse adjusts the width of an external fly-back pulse and subsequently generates a horizontal blank signal within the video signal processing unit where a video amplifier, an on-screen display unit and horizontal/vertical synchronous processing unit have been realized as one chip. The method of adjusting the width of a fly-back pulse includes the steps of shaping the fly-back pulse received from the outside, determining whether a mid-point of the shaped fly-back pulse is detected, generating a lamp signal by charge pumping performed corresponding to a section of the fly-back pulse if the mid-point of the shaped fly-back pulse is detected, comparing the lamp signal with a predetermined reference voltage, thereby generating an output signal from the result of the comparison, and combining the comparison output signal and the shaped fly-back pulse, thereby generating a horizontal blank signal.